In radio systems, such as mobile systems, a radio signal between a mobile phone and a base station propagates along several routes between a transmitter and a receiver. The signal may propagate directly from the mobile phone to the base station, provided there are no obstacles between them. In urban environments, buildings, cars and other obstacles cause reflection and scattering of the radio signal. Multipath-propagated components of a signal may thus propagate distances of various lengths on the radio path, resulting in that the components arrive at the receiver at different times. Some radio systems, such as radio systems implemented by a spread spectrum technique and employing code division multiple access (CDMA), can utilize this multipath-propagation. In such a case, the receiver receives each multipath-propagated signal component, and by amplifying and combining the components, the transmitted original signal can be better identified.
In CDMA, each signal comprises an individual spreading code which modulates the baseband while spreading the frequency band of the data signal. Data signals of several users are transmitted simultaneously on one and the same frequency band and the users are distinguished by the spreading code. Correlators in receivers synchronize themselves to a desired signal which they identify by the spreading code, and return the frequency band of the signal to the original one. Signals, which contain another spreading code, arriving at a receiver do not, in an ideal situation, correlate but keep their wide frequency band and are thus received as noise in the receivers. The aim is to select the spreading codes used by the system so that they are orthogonal with respect to each other, i.e. do not correlate with each other. One user can have one or more spreading codes depending on the required transmission capacity.
A RAKE receiver made up of one or more RAKE fingers, i.e. correlators, is generally used as a CDMA receiver. RAKE fingers are independent receiver units whose task is to despread and demodulate one received multipath-propagated signal component. In addition to the RAKE fingers intended for receiving signals, a CDMA receiver typically has at least one separate searcher whose task is to search for the various signal components transmitted with a desired spreading code, to identify their phases and to allocate the signal components to the RAKE fingers. The searcher is implemented according to prior art, for instance by means of a matched filter (MF). In practice, the length of a matched filter in a searcher finger is 256 spreading code units, i.e. chips, because the phase of the received signal is not known. Each RAKE finger can be directed to correlate with a signal component propagated along a different route, each component arriving at the receiver delayed in a slightly different manner. The RAKE fingers are directed by giving the correlator information on the desired spreading code and its phase.
After starting to receive a signal, according to prior art, a RAKE finger keeps its spreading code synchronized to the incoming signal of the finger using the known early-late code tracking loop, for instance. The receiver then has three correlators: one tracks the incoming signal exactly synchronized to it, a second synchronizes itself to the early phase of the spreading code, which is a phase preceding the current phase by half a chip, for instance, and a third synchronizes itself to the late phase which is a phase delayed by half a chip from the current phase.
A receiver solution in which each RAKE finger tracks the changes in the delay of its own code phase, has significant drawbacks, because in connection with RAKE fingers, the implementation of the correlators and tracking the delay increase considerably the complexity of the implementation of the finger. A further considerable drawback in prior art solutions is that when the fingers track independently their signal components propagating in different directions, the components have a tendency to merge, whereby two different fingers synchronize themselves to the same spreading code phase. One prior art solution, a matched filter of a traffic channel, which is relatively long, has increased the complexity of the receiver of the searcher on account of the required computational power, thus also increasing the requirements set on the equipment.